Oh, he had no doubts at all, after all these years. Kirk was one of the people he'd come back to time and again, one of the few who really understood the lure of sex without strings, without a strong emotional commitment, just for the pleasure of it. They'd grown closer and forged a good bond over that time - they had more in common than just impressive sex drives, after all - but he'd never fallen in love with Kirk, and that made this so much easier, so much cleaner than what he'd had with Ianto.
"Is that a challenge?" he murmured, hands smoothing down Kirk's back, exploring the faint differences from the man he'd fucked last night. Nothing that noticeable, just skin that was slightly tauter and smoother, a little less padding around the middle. Nothing different about the enthusiasm of the kiss, though. "Because you ought to know, I kept pace with a couple of vampires for years." Sliding his hands down to Kirk's ass, pulling him against him to try and get more of those lovely gasps.